Internal combustion engines operate on the principle of a vacuum induced flow of fuel through a carburetor and into the engine. The carburetor is where air mixes with the fuel to create a flammable vapor or mist that is ignited in the engine. Carburetors are typically self-regulating systems where fuel becomes atomized into an airstream. When the engine runs at higher revolutions per minute (rpm) a greater vacuum is created, causing more air to flow through the carburetor and thus more fuel to be sucked into the air stream. A venturi, or venturi passage, is a channel within the carburetor where air flows to form a relatively low pressure, causing atomization of the fuel being fed to it.
In the prior art, venturi passages are circular in cross section. This shape provides a rather uniform volume through which the air passes. Fuel is typically fed to the venturi from a fuel float chamber by way of a small fuel line. The vacuum created in the venturi pulls the fuel through the line, and when the fuel reaches the venturi, the airstream and low pressure cause the fuel to atomize into a fine spray which is fed to the engine for combustion. For larger engines, such as a single cylinder, large displacement engine or a two cylinder large displacement engine, the prior art sometimes uses two carburetors in order to feed it enough air-fuel mixture.
The air-fuel mixture supplied by the carburetor can be adjusted by way of a throttle. A throttle is a device which can be adjusted to block a desired amount of the carburetor's air passage, thereby limiting the amount of air and fuel that reach the engine.
Fuel injectors are also used in the prior art to supply air-fuel mixtures to engines, and usually can deliver more accurate amounts of fuel. With fuel injectors, the speed of the airstream does not have as much of an effect on the amount of admitted fuel as naturally aspirated carburetors.